EU leaders grapple with the threat of new coronavirus variants Thursday as wary countries push for travel restrictions on Europe's internal borders to stop the spread.

The chiefs will hold a summit -- by videolink to protect themselves from infection -- to address "the seriousness of the situation with the new variants," an EU official said.

Virus mutations spreading from Britain, South Africa and Brazil have alarmed EU authorities because of their increased infectivity, prompting bans or restrictions on travellers from those countries.

Now calls are increasing to shut the intra-EU borders to slow their spread, but German Chancellor Angela Merkel said Thursday that such a step would be "the last resort".

The goal first is to bring down the high infection rates in all EU countries, she said, noting that "if a country with a rate perhaps twice as high as Germany opens all its shops and we still have them closed, then of course you have a problem".

If that is not possible, border controls "cannot be completely ruled out," she said.

She and other leaders stressed the need for coordination, though, to avoid the experience in March when several member states panicked and closed off national borders unilaterally, triggering travel chaos.

That decision came to be seen as disastrous, disrupting the already stumbling European economy, and the leaders will work hard to find ways to thwart the variants while keeping factories and businesses running.

Belgium -- wedged between Germany, France and the Netherlands -- said it would plead for a "temporary" closure during the February-March holiday period, when it celebrates a Carnival season and millions of Europeans usually head for the ski slopes.

In an interview with AFP a few hours before the summit, Foreign Minister Sophie Wilmes said Belgium wants to ban "non-essential mobility" outside the country's borders until at least the end of the February holidays.

"We have to understand that we are in a totally exceptional situation," she said, adding that it would be "peculiar" to tell Belgian bars and restaurants to close but allow tourists to head abroad.

Ambitious' inoculation plan 

For now, the variants remain a tiny proportion of overall cases in the EU, and health officials are racing to give vaccination jabs before the mutants dominate.

While there was no indication as yet the new variants were more deadly, there were concerns they could infect more people and overload hospital intensive care capacity. 

European Commission chief Ursula von der Leyen said the bloc will soon expand vaccines beyond the BioNTech/Pfizer and Moderna ones currently authorised to inoculate 70 percent of adults in the EU before September.

"This is doable, it is ambitious, but we have to be ambitious -- people are waiting for that," she said Wednesday.

The vaccination roll-out across the European Union has been disappointingly slow compared with the United States, Israel and other countries, a problem compounded by delivery shortfalls of the BioNTech/Pfizer doses. 

There is also a minority of people, particularly in countries such as France, who hesitate to be inoculated.

And, while many hope the jabs will soon put an end to limited travel, tests, nighttime curfews and home quarantines, EU officials and diplomats warn against Europe lowering its guard too early. 

Vaccination certificates

The leaders are also expected to discuss the idea of using vaccine certificates as a form of travel passport in Europe, as championed by tourism-dependent Greece, but diplomats insisted this topic was at its early stages.

"There are still many, many, many questions to be answered. First of all, we don't know if people who have been infected can still be contagious and can still infect other people," a senior EU diplomat said.

One concern for EU officials is seeing Europe divided between a small minority able to travel and enjoy post-pandemic life while others remain confined.

The summit was to also talk about moves to have antigen tests that are cheaper and less invasive -- but less reliable -- than the nose-probing PCR swabs that are currently widespread.

In the meantime, the EU will have to ramp up genome analysis of virus samples to detect the spread of the new variants.